1. Field of the Invention
The present invention relates generally to optical systems. The present invention relates more particularly to optical systems used for projection displays communication systems.
2. Description of the Background Art
A two-dimensional projection image may be formed by using one or more linear arrays of light-modulating elements. The light-modulating elements may comprise, for example, GRATING LIGHT VALVE (GLV) elements.
Publications describing GLV devices and their application to display systems include: “The Grating Light Valve: Revolutionizing Display Technology,” by D. M. Bloom, Projection Displays III Symposium, SPIE Proceedings, Volume 3013, San Jose, Calif., February 1997; “Grating Light Valve Technology: Update and Novel Applications,” by D. T. Amm and R. W. Corrigan of Silicon Light Machines in Sunnyvale, Calif., a paper presented at the Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; “Optical Performance of the Grating Light Valve Technology,” David T. Amm and Robert W. Corrigan of Silicon Light Machines, a paper presented at Photonics West-Electronics Imaging, 1999; “Calibration of a Scanned Linear Grating Light Valve Projection System,” R. W. Corrigan, D. T. Amm, P. A. Alioshin, B. Staker, D. A. LeHoty, K. P. Gross, and B. R. Lang, a paper presented at the Society for Information Display Symposium, May 18, 1999, San Jose, Calif.; “An Alternative Architecture for High Performance Display,” R. W. Corrigan, B. R. Lang, D. A. LeHoty, and P. A. Alioshin of Silicon Light Machines, a paper presented at the 141st SMPTE Technical Conference and Exhibition, Nov. 20, 1999, New York, N.Y.; and U.S. Pat. No. 6,215,579, entitled “Method and Apparatus for Modulating an Incident Light Beam for Forming a Two-Dimensional Image,” and assigned at issuance to Silicon Light Machines. Each of the above-mentioned publications is hereby incorporated by reference in its entirety.
In such display systems, the linear array modulates an incident light beam to display pixels along a column (or, alternatively, a row) of the two-dimensional (2D) image. A scanning system is used to move the column across the screen such that each light-modulating element is able to generate a row of the 2D image. In this way, the entire 2D image is displayed.
FIG. 1 is a diagram depicting the reflective and diffractive operational states of a GRATING LIGHT VALVE (GLV) element. The left side of the diagram depicts the reflective (dark) state, while the right side of the diagram depicts the diffractive (bright) state.
In the example illustrated in FIG. 1, the substrate may comprise a silicon substrate with oxide (for example, about 5000 angstroms thick) overlaying the silicon, and tungsten (for example, about 1000 angstroms thick) overlaying the oxide. The reflective members lie above the tungsten with an air space there between. For example, three pairs of reflective members (i.e. six members) are shown. The reflective members may, for example, comprise reflective ribbons comprising nitride (for example, about 1000 angstroms thick) with a reflective layer of aluminum (for example, about 500 angstroms thick) on the nitride. Incident light is beamed onto the reflective members. The incident light beam may be at a perpendicular angle to the plane of the substrate.
In the reflective or dark state (left side), all the reflective members are in the same plane, and the incident light is reflected from the surfaces of the members. This reflective state may be called the dark state because it may be used to produce a dark spot (dark pixel) in a projection display system. Such a dark pixel may be produced by blocking the light that is reflected back along the same path as the incident light.
In the diffractive or bright state (right side), alternate ones of the reflective members are deflected downward. This results in the diffraction of the incident light in a direction that is at an angle to the path of the incident light. This reflective state may be called the bright state because it may be used to produce a bright spot (bright pixel) in a projection display system. Such a bright pixel may be produced because the angularly reflected light is not blocked. As discussed further below, the optical response of the element depends on the amount of downward deflection of the alternate members.
FIG. 2 is an illustration depicting a GLV element comprising pairs of fixed and movable ribbons. As depicted in FIG. 2, the GLV element may include pairs of reflective ribbons, each pair having one fixed and one movable ribbon.
FIG. 3 is a diagram depicting deflections of reflective members for a GLV element in a diffractive state. The GLV element comprises a plurality of reflective members 302. In the example illustrated, the GLV element includes three pairs of reflective members 302 (i.e. six of them).
In the diffractive state, the reflective members are controllably arranged in an alternating configuration at two heights (304 and 306). A first height 304 may correspond to reflective members at a fixed height, while the second height 306 may correspond to reflective members pulled down by application of a voltage.
The grating plane 308 is a theoretical plane that corresponds to the plane on or about which the reflective members are aligned. As illustrated in FIG. 3, the incident light beam 310 impinges upon the element at an angle perpendicular to the grating plane 308. Diffracted light 312 travels away from the element.
The difference between first and second heights may be defined as the deflection distance γ. The amount of the deflection γ may be varied by application of different voltages to control the amount of incident light reflected from the element. When γ is near zero, the element would be near a maximally reflective state. When γ is near λ/4, where λ is the wavelength of the incident light, the element would be near a maximally diffractive state.
FIG. 4 is a graph illustrating a non-linear electro-optic response for a GLV element. The graph shows intensity of light (in arbitrary units) versus voltage. The higher the voltage is, the larger is the displacement γ of the element. Depending on the voltage applied to the alternate reflective members, the light intensity varies. For the most part, the higher the applied voltage, the higher the light intensity. (This relationship reverses for sufficiently high voltages where the light intensity reduces with higher voltages, and hence the downward slope of the graph at the far right.)
One challenge to overcome in order to implement a high-performance projection display using GLV technology relates to calibration of the display system. Without calibration, the image may be non-uniform due to variations in the source illumination, the system optics, and the GLV elements.